MONTREAL — Tumour size may not be the best method of predicting the progression of some inherited breast cancers, suggests a McGill University study.

"We have identified a group of breast cancer tumours that don't conform to previous observations made in the general population of women with breast cancer," Dr. William Foulkes, a McGill geneticist and lead investigator, said in a news release.

"For these tumours there is only a weak correlation between tumour size, the local spread of cancer cells and the likely severity of disease."

Foulkes and his colleagues studied more than 1,500 women with breast cancer at 10 North American centres between 1975 and 1997.

The findings will be published in next month's issue of Cancer magazine.

The women with a mutation in the BRCA1 gene had tumours which didn't behave as expected since there was no clear correlation between tumour size and cancer in the axillary lymph nodes.

This isn't true for breast cancers in the general population or those related to another gene called BRCA2.

Foulkes said the findings have important implications for early diagnosis and treatment of women who carry the BRCA1 gene. Jewish women from Eastern European backgrounds have a higher probability of carrying the gene.